Loaders are used to lift and move bulk materials. A loader comprises a self-propelled vehicle having a loader linkage and associated actuators for controllably moving the loader linkage. Such loaders may be provided with wheels or endless tracks for propulsion. A typical work cycle for a loader involves loading material from a pile into the loader bucket, then moving the vehicle to dump the material at a new location, and then repeating. During the bucket loading operation, it is customary for the loader boom be lowered and for the loader bucket to be driven into the pile by the forward movement of the vehicle.
The motive force of the vehicle acting on the pile during this operation generates a crowd force against the loader linkage, increasing the actuator force required to raise the loader linkage and subsequently increasing the cycle time. If sufficiently high, the crowd force may overpower the linkage actuators, preventing upward movement of the loader linkage altogether. In this situation, the operator is required to manually reduce the vehicle motive force in order to reduce crowd on the loader linkage, further increasing cycle time. The increased cycle time and operator effort attributed to crowd force diminishes overall productivity.
A number of mechanisms and systems have been used to limit crowd force in loaders and similar machines. Examples that control crowd force reduction by sensing resistance of upward movement of the loader linkage and then reducing power output are disclosed in U.S. Pat. Nos. 3,749,269 and 4,776,751. Various additional examples that control crowd force reduction by coupling the hydraulic lift and tilt circuits of the loader linkage to the drive train to reduce power output are disclosed in U.S. Pat. Nos. 3,583,243, 3,796,336, and 4,015,482.